This invention pertains to abrasion-resistant cementitiously hardened products and to methods of making such cementitiously hardened products. Flue gas desulfurization (FGD) sludge is incorporated in the composition of the present invention, and the product is a synthetic aggregate.
The concept of removing sulfur oxides from the exhaust of coal-burning power stations by exposing those exhaust gases to an aqueous suspension of lime or limestone is well known. The by-product of this process, FGD sludge, is a problem. Disposal and contamination of groundwater by FGD sludge are primary concerns.
In a well known disposal method, FGD waste is interblended with fly ash, often derived from the same power station, and lime to form a cementitious mixture. The resulting mass, which typically has a damp soil-like consistency, is conveyed to a landfill site, where it is spread and compacted. Pozzolanic and sulfopozzolanic reactions take place in the compacted material to generate a load bearing capability which is measured in the laboratory as unconfined compressive strength. In addition, the pore space within the compacted mass is reduced as reaction products fill the voids; this reduces the permeability coefficient, thereby reducing possible percolation of rainwater into the water table carrying dissolved species. It has also been proposed that the inter-mixed cementitious mass be used to form synthetic aggregate (see, for example, U.S. Pat. No. 3,785,840).
Attempts to make synthetic aggregate from FGD waste-fly ash-lime compositions have been unsuccessful due to the relative softness or friability of the aggregate so generated. An approach for improving permeability coefficients and, incidentally, unconfined compressive strength of hardened products, formed from such compositions, is disclosed in U.S. Pat. No. 4,613,374 (of common assignment and common inventorship herewith).
Known (to applicants) previous attempts to use cementitiously hardened FGD sludge-fly ash-lime compositions as synthetic aggregate have been unsuccessful, largely due to softness of the aggregate produced even after the material is removed from cured landfills where it has been placed under compaction. While the unconfined compressive strength may be on the order of 200 to 300 psi after a long curing time, the aggregate is still relatively soft and performs poorly in the Los Angeles abrasion test for measuring aggregate hardness (or abrasion resistance).
The formation of ettringite, a tricalcium alumino sulfate hydrate (3CaO.Al.sub.2 O.sub.3.3CaSO.sub.4.31H.sub.2 O), also presents problems for cementitiously hardened products produced by the previous methods. The presence of ettringite in significant quantities is known to be destructive in Portland cement concrete. It is this mechanism that causes ordinary concrete to fail in the presence of sea water. This failure is due to expansion: The formation of ettringite within a mass causes expansion, largely due to the 31 molecules of water that are taken into the crystal formation of ettringite.
Although this destructive expansion is not normally seen in landfill disposal of FGD sludge-fly ash-lime compositions, it has been noted as a destructive force when this composition is utilized as a road construction base course. The same destructive force has been noted in laboratory sized Proctor specimens, in which FGD sludge-fly ash-lime compositions are prepared and compacted into Proctor cylinder molds, 4" in inside diameter and 4.5" in height. Formed specimens are then ejected from the molds for determination of density, unconfined compressive strength, etc. Careful measurements of the cylindrical specimens having ettringite have indicated growth of 5%, accompanied by severe cracking and structure failure due to internal stress release.
Ettringite forms due to the presence of calcium sulfate dihydrate, identified as gypsum, which is almost always present to some degree in FGD sludge. Assuming the coal combustion unit is operating with combustion zone dynamics limiting the level of oxygen present, a less oxidized form, calcium sulfite hemihydrate, will form. Given any reasonable degree of excess oxygen in the combustion system, which typically operates at 5% to 7% excess, some quantity of gypsum will be formed from the calcium sulfite hemihydrate during or after the scrubbing unless an oxidation inhibiting agent is added. The gypsum level is typically from about 1 to about 15% of the total alkaline earth metal-sulfur oxide compounds, with the remainder as calcium sulfite hemihydrate. In certain scrubbing operations, deliberate oxidation of the alkaline earth metal-sulfur compounds is forced by excessive air circulation through the scrubbing modules. In summary, all lime or limestone scrubbing units on pulverized coal boilers yield some quantity of calcium sulfate dihydrate or gypsum; this may range from about 1 to 100% but generally will be at least 10 % (by weight) of the sludge solids. Thus a problem with expansion in the cementitious hardening of formed or confined masses of a lime-fly ash-FGD sludge would normally be expected, almost regardless of the FGD sludge source